1. Field of the Invention
The present invention relates to a coding/decoding apparatus having a plurality of coding modes and used for an image signal.
2. Related Background Art
Various coding schemes have been proposed to reduce the transmission data quantity in performing digital transmission of image information.
According to one of the proposed coding schemes, coding is performed by switching intraframe coding (intra) and interframe coding (inter). Intraframe compression is a scheme of reducing the information quantity by utilizing the characteristic of an image that adjacent pixels are similar in brightness and color. In compressing an actual image, since most parts of the image, e.g., sky and wall portions, are almost similar in brightness and color, the data quantity can be reduced to about 1/5 to 1/10 by using only the intraframe compression scheme.
Interframe compression is a scheme of obtaining an image from only information of a correction portion by using similar images.
In general, adjacent frames of a motion image similar in graphic pattern although there is a slight motion or change. Using this point, similarities (motion, color, brightness, and the like) between a frame to be compressed/coded and an adjacent frame are calculated. Values for a frame similar to the "frame to be coded" are calculated from "prediction values", i.e., the "adjacent frame" on the basis of the calculations.
Subsequently, only difference information between the "prediction values" and the corresponding values of the frame to be coded is coded (recorded/transmitted). For this reason, the data quantity (correction portion) is reduced.
Assume that a motion image includes only a human figure, and the human figure moves to the right. In this case, pixels corresponding to the human figure in a preceding frame, including correction information of movement, are prediction values, and differences are obtained by subtracting the prediction values from the values of all the pixels which have moved to the right.
It is known that in general, in a conventional coding apparatus, when an error occurs on a transmission line in performing compression by inter-coding, the error propagates. Therefore, intra-coding is automatically performed every time inter-coding is performed a predetermined number of times.
As described above, in the above conventional coding apparatus, when inter-coding is continued, the above refresh operation is performed at predetermined time intervals to prevent continuous propagation of an error caused in the process of transmission. If, however, an error occurs in intra-coded information, propagation of the error cannot be prevented during a period before intra-coding or a refresh operation is performed.